1. Field of the Invention
The present invention relates to a speech coding apparatus, speech decoding apparatus and speech coding/decoding method in sub-band ADPCM (Adaptive Differential Pulse Code Modulation).
2. Description of the Related Art
Conventionally, as a speech coding apparatus and speech decoding apparatus used in sub-band ADPCM, there are known apparatuses conforming to ITU-T (International Telecommunication Union Telecommunication sector) Recommendation G.722.
FIG. 1 is a block diagram illustrating configurations of speech coding apparatus 300 and speech decoding apparatus 400 used in two-sub-band ADPCM described in Recommendation G.722.
Speech coding apparatus 300 is comprised of 24-tap splitting filter bank 310 that splits a frequency band of an input signal to two sub-bands and outputs sub-band signals, ADPCM quantizers 320a and 320b that quantize respective two-split-sub-band signals, and multiplexer 330 that multiplexes codewords quantized in ADPCM quantizers 320a and 320b to produce a bit stream.
Meanwhile, speech decoding apparatus 400 is comprised of demultiplexer 410 that outputs codewords for each sub-band obtained from transmitted data streams, ADPCM dequantizers 420a and 420b that dequnantize respective codewords for each sub-band output from demuletiplexer 410 to output sub-band signals, and 24-tap synthesis filter bank 430 that performs synthesis filtering on the sub-band signals.
Operations of speech coding apparatus 300 and speech decoding apparatus 400 each configured as mentioned above will be described below.
A frequency band of an input signal is split to two sub-bands in splitting filter bank 310 and two sub-band signals are generated. Each of the sub-band signals is assigned a predetermined number of quantizing bits and quantized in respective one of ADPCM quantizers 320a and 320b. The codewords obtained by quantization are multiplexed in multiplexer 330 to be bit streams.
Meanwhile, in speech decoding apparatus 400, the bit streams with a plurality of multiplexed codewords are demulitiplexed in demultiplexer 410 to be codewords for each sub-band. The codewords for each sub-band obtained by demultiplexing are dequantized in ADPCM dequantizers 420a and 420b to be sub-band signals. The sub-band signals are subjected to synthesis in synthesis filter bank 430 to be a decoded signal.
However, in the conventional speech coding apparatus and speech decoding apparatus as described above, since the number of quantizing bits is fixed which is assigned to each sub-band signal in an ADPCM quantizer in the speech coding apparatus, in particular, when a sampling frequency of an input signal becomes high, there is a risk that the bit assignment is not optimal and that audio quality of decoded signals may deteriorate in the speech decoding apparatus.